I've noticed a few things regarding appearances in DEF2 1 that seem to be errors.

1) From what I can see from looking at the pages and not reading them, the only non-Defenders who actually make an appearance are the FF on page 11 Panel 3. Everyone else would seem to warrant a BTS credit from page 11 panel one when Wong states, "The Avengers are already facing these demons -- the Fantastic Four and Spider-Man as well --"

So it looks like the Avengers who are credited with an appearance, Captain America and Iron Man for example, should have a -BTS next to the DEF2 1 in their timelines.

EDIT: Whoops! I was mistaken. Captain America and Iron Man do appear on a TV screen on page 23. Also, on page 19 there are captions suggesting more BTS appearances. "He twitched a hand in a slight mystic gesture and Warlord Kaa and his Shadow Warriors invaded Ontario, attacking the super-team called Alpha Flight. Another twitch and a squad of Living Erasers appeared in the Tokyo headquarters of Big Hero Six. Another, and Genosha is inundated with Lizard-Men of Tok."

2) There's no mention of DEF2 1 anywhere on Spider-Man's timeline. Where that might occur leads me to #3.

3) According to current timelines, the Human Torch's appearance in PPSS 37 comes before DEF2 1

It looks like the problem is with the Human Torch. If his appearance in PPSM 37 is moved after his BTS in A3 43, that should take care of it. As for where DEF2 1 -BTS should go in Spider-Man's timeline, it would need to be before DD2 24 at least.

EDIT: It seems I spoke too soon regarding where PPSM 37 should land in Torch's timeline. I'll eventually write another post regarding the conflicting placements of Infinity Abyss in various timelines (I think I should read it before opening my mouth again), but suffice it to say, because of Spider-man's and Captain America's appearances in that series, and Iron Man's appearance in FF '01, the Human Torch appearance in PPSM 37 should come between DEF2 1 and FF '01.